1. Field of the Invention
The invention relates to cooling systems in which plural liquid cooled units are part of a single coolant circulation system, and more particularly to means for minimizing damage to the cooled unit in the event of leakage in one.
2. Prior Art
In many complex electronics systems, large amounts of heat are generated in confined spaces. Current systems invariably use solid state active components which generate heat and which cannot operate without such heat being removed. For solid state components, a rise in temperature produces an alteration in performance; destruction, if the temperature rise is too great; and if the temperature rise is sustained, an acceleration of the time between failures. In aviation electronics (i.e. "avionics") the problem is particularly acute, since weight and volume must be conserved, and since there is extreme concern for reliability. The solution for cooling avionics units is increasingly liquid cooling, which allows for denser packaging.
In seeking maximum reliability, the answer is redundancy for such primary functions as flight control and engine control and triple or quadruple redundancy is common. In such systems, three or four flight control computing units operate continuously, all providing simultaneous computations to a common decision logic, which maintains a check on the "health" of the units, and decides which computations to rely on.
With redundant avionics units, all operating, and space and weight being at a premium, cooling of all units is essential, preferrably by efficient liquid cooling. In addition, the cooling must be provided in a manner allowing maximum reliability.
The practical solution for cooling multiple avionics units has evolved to a single coolant circulation system having a single circulating pump and refrigeration unit supplying cooled liquid in tubing connected to each of the multiple units. While there are many practical advantages in using a single circulation system, there is a disadvantage in that a leak in one of the units may not only subject the electronics in the leaking unit to damage from overheating, but may drain coolant from the entire system, and place the electronics of all the units at risk.
The potential causes of leakage are multiple and the leaks may be either of a slow nature or catastrophic. Slow leaks may arise from a fitting that has vibrated loose, a crack or failure in the piping or in the ducts carrying the coolant through the avionics unit. A catastrophic failure may result from loss of a turbine bucket in an engine failure, or damage to the aircraft which ruptures a piece of tubing.
A primary concern in protecting against leakage are that the means selected to detect the leak be reliable. Reliability in this context includes having sensitivity to small leaks, being able to discriminate between a leak and other transient affects, and being readily calibrated. One solution that was investigated was to sense the pressure drop across an avionics unit using a mechanical isolation valve, designed to trip when the pressure drop across the unit changed due to a leak. The change in pressure drop produced by a leak of a given size depends upon the pumping rate, pumping pressure, and the impedance of the unit to the flow of coolant. Pressure sensing tended to detect only the larger leaks that would produce a significant change in the pressure drop across the unit. The normal pressure drop, furthermore was not stable under ordinary conditions. As a result the system was very hard to calibrate for discrimination between leaks and other effects causing changes in the pressure drop across the unit. Unsuccessful attempts to eliminate the problems in the pressure measurement approach led to the development of a more reliable method of leak detection.